fake_outputs: Allow designating a fake output as primary
fake_outputs: Allow designating a fake output as primary

Allow appending 'P' to the fake output specification to set the
created output's "primary" flag, to allow writing test cases that
depend on the presence of a primary output.

Airblader/i3,Airblader/i3-original,i3/i3,Airblader/i3-original,Airblader/i3-original,stapelberg/i3,cornerman/i3,cornerman/i3,Airblader/i3,i3/i3,i3/i3,Airblader/i3,stapelberg/i3,Airblader/i3-original,cornerman/i3,stapelberg/i3,stapelberg/i3,Airblader/i3,Airblader/i3-original,cornerman/i3,i3/i3